Integrated circuits are made in a bulk parallel process by patterning and processing semiconductor wafers. Each wafer contains many identical copies of the same integrated circuit referred to as a “die.” Semiconductor wafers must be tested before the die is cut into individual integrated circuits and packaged for sale. If defects are detected the defective die can be culled before wasting resources packaging a defective part.
To test a wafer, a probe card is commonly used which comes into contact with the surface of the wafer. The probe card generally contains three unique characteristics: (1) an XY array of individual probes that bend in the Z direction to allow contact with the die; (2) an electrical interface to connect the card to a circuit test apparatus; and (3) a rigid reference plane defined in such a way that the probe card can be accurately mounted in the same location on the wafer tester. FIG. 7 illustrates an array of probes (705) on a substrate (710). Probe tips (715) for each probe in the array (705) are allowed to bend in the Z direction (perpendicular to the substrate (710)). When the probe card is brought in contact with the die, the Z-direction bending allows for a solid contact with the probe tip (715). The probe card ultimately provides an electrical interface that allows a circuit test apparatus to be temporarily connected to the individual die. This method of die testing is extremely efficient because many die can be tested at the same time. To drive this efficiency even higher, probe card manufactures are making larger probe cards with an ever-increasing numbers of probes.
FIG. 8 depicts a cross sectional view of a probe card showing the substrate (805), probe base (810), bending element (815), tip supporting structure (820), and probe tip (825). The entire probe card is generally moved in the Z-direction (depicted by arrow 830) causing the bending element (815) to bend allowing the probe tip (825) to come into contact with the die that is under test. FIGS. 9A to 9C illustrate how the probe bends while being brought into contact with the die. FIG. 9A depicts a cross sectional view a probe card with showing the substrate (905), probe base (910), bending element (915), tip supporting structure (920), and probe tip (925). FIG. 9A is the starting position of the probe—i.e, no bending of the bending element (915). Between the probe bending element (915) and the substrate (905) is a clearance (930), often called the overdrive. Die manufacturers often specify the overdrive for the probe cards that test the die. Referring to FIG. 9B, the probe tip (925) is moved in the direction of arrow 935 in contact with the die (not shown) causing the bending element to bend (shown as 940) and also causing the clearance to decrease (shown as 945). At this position, the probe card should stop and allow the testing of the die to begin. However, if the probe card is inadvertently brought too close to the die, the bending element will bend even further until it hits the substrate (shown as 950 in FIG. 9C), an event herein referred to as excess overdrive. This may damage the probe and, possibly the die it is testing. The damaged probe card and/or die may yield false positives, reducing the testing line yield.
What is needed, therefore, is a method and apparatus that allows for quick and inexpensive inspection to identify probes that have experienced excess overdrive on a probe card.